The present invention relates to methods for clarifying coal liquors or slurries by employing water-soluble polymers.
Clarification of coal liquors or slurries is an important aspect in the economy of operating a coal production plant. Generally, such plants are operated on a closed system basis in that the water used to wash the raw coal is recycled for reuse. Since such water serves primarily to remove coal fines and clays mixed with the raw coal thereby forming coal liquors, it is critical that the water be relatively clear and contain only minor amount of suspended solids when it is reused in the washing procedure. Otherwise, undesirably large volumes of water would be necessary to process the coal. Since the coal fines and clay which are removed from these coal liquors are often used as filler materials and in other applications which require inexpensive inert mateials, it is essential that such coal liquor clarification be very inexpensive.
Previously, the coal industry has employed various anionic and cationic water soluble polymers to remove both coal particles and colliodal clay particles. For example, as taught in U.S. Pat. No. 3,408,293, it is a common practice to prefloc the coal liquor or coal slurry with anionic polymers in order to coagulate the coal particles and subsequently treat the prefloc slurry with cationic polymer in order to flocculate the coal particles. Unfortunately, as indicated in this patent, very careful control over the quantities of anionic and cationic polymers employed must be exercised in order to obtain efficient clarification of the coal liquor. Also, significant quantities of the various polymers need to be employed in order to achieve such desirable coal liquor clarification.
Alternatively, U.S. Pat. No. 4,141,827 teaches the use of a low molecular weight polyacrylamide which has been chemically modified to provide quaternized dimethylaminomethyl groups on the amide groups for settling suspended solids in coal washing waste water. The specified acrylamide polymer is prepared by aminoalkylating the amide moiety with a dialkylamine and formaldehyde and subsequently quaternizing their reaction product. The polymer for use in the process has a molecular weight of about 7,000 to 110,000 and at least about 50 mole percent of the acrylamide mer units are quaternized. Due to the fact that the pendant amide groups will cross-link the polymer via methylenebis moieties in the presence of formaldehyde, it is necessary to maintain a relatively high conversion level in the Mannich reaction, i.e., the amide functionality must be maintained as low as possible. Preferably, at least 60 to 90 mole percent of the acrylamide groups have been quaternized. In the examples, formaldehyde and dimethylamine are employed in a stoichiometric excess in relation to the polymer to quaternize the maximum amounts of acrylamide groups.
In view of the foregoing deficiencies of the prior art processes for clarifying coal liquors, it remains highly desirable to provide a method for coal liquor clarification which does not exhibit the deficiencies of the prior art processes.